The present invention relates to an adhesive usable at ambient temperature to bond together two cross-linked rubber compositions, to a process for preparing the adhesive, to a tread of a tire cover and to such a cover comprising this adhesive, and to an article comprising two parts securely bonded together by means of this adhesive. The invention applies in particular to the tire retreading industry, wherein a cross-linked tread is bonded to a cross-linked tire casing (known as xe2x80x9ccoldxe2x80x9d retreading).
Known adhesives for bonding together two cured elastomeric articles, such as the casing of a tire cover to be retreaded and a cross-linked tread, are often based on polyurethane or polyurea type polymers.
A polymeric adhesive which is produced by reacting a polyurea prepolymer, which may comprise urethane groups and which ends with isocyanate groups, with a binary blend consisting of a particular polyether amine (polyoxytetramethylene bis(p-aminobenzoate)) and of a chain extender consisting of an aromatic diamine, is disclosed in U.S. Pat. No. 5,183,877. The polyurea prepolymer is obtained in gel form by reacting a polyisocyanate, such as a methylene diisocyanate modified with a carboimide (abbreviated as CD-MDI), with a polyether amine such as the one described. A major drawback of the adhesive described in U.S. Pat. No. 5,183,877 lies in the use of the prepolymer for its preparation. Specifically, the high viscosity of the prepolymer inevitably involves using the aromatic diamine chain extender and bringing the prepolymer to a temperature of 40xc2x0 C. in order to go from its gel state at ambient temperature, to the liquid state in order to use as an adhesive.
The Inventors have unexpectedly found that a copolymer comprising isocyanurate groups combined with urea or urethane groups, which is a reaction product of:
1) a polyisocyanate comprising isocyanurate rings, having a functionality equal to or greater than three,
2) a functional polymer comprising an amine or alcohol group at each of its chain ends, the polymer being selected from the group consisting of polyether diamines, polyether diols, polyester diols and (ether-ester) diol copolymers, and
3) at least one trimerization catalyst capable of forming isocyanurate rings from isocyanate groups;
wherein the molar ratio of the total number of moles of isocyanate groups to the total number of moles of amine in the copolymer is between 1.5 and 2.2,
may be used as an adhesive for the satisfactory bonding together at ambient temperature of two cross-linked rubber compositions, because the copolymer exhibits reduced viscosity at ambient temperature.
The presence of the isocyanurate rings in the copolymer thus provide the adhesive thermal stability at temperatures above 100xc2x0 C., which may be reached, for example, by a tire cover under driving conditions.
In addition, the adhesive of the invention has a relatively long xe2x80x9copen timexe2x80x9d (time limit for use, also known as the xe2x80x9cpot lifexe2x80x9d). As a guide, this xe2x80x9copen timexe2x80x9d ranges from 5 min to 15 min.
Polyisocyanates with a low viscosity at ambient temperature, preferably those having a viscosity, measured according to the Brookfield technique, of between 8 and 16 Poises, are used for the present invention. The polyisocyanates may be aliphatic or aromatic and may be dimers or trimers. In accordance with a preferred embodiment of the invention, a trimer derived from hexamethylene diisocyanate having a functionality of equal to or greater than three is used as polyisocyanate.
A polyether diamine, which is intended to react with the polyisocyanate to obtain the urea groups may be used as the functional polymer. Examples include a polyoxypropylene diamine, a polyoxyethylene diamine and, preferably, a polyoxytetramethylene diamine, such as polyoxytetramethylene bis(p-aminobenzoate).
Additionally, the polyether diamine may be a polyether comprising an aliphatic amine group at each chain end, such as a polypropylene glycol diamine or a polytetrahydrofuran diamine (obtained by reacting anthranilic acid with polytetrahydrofuran, whose molecular mass can range from 650 to 2000 g/mol).
An ether and/or ester diol polymer, which is intended to react with the polyisocyanate to obtain the urethane groups, may also be used as functional polymer. Exemplary polyether diol include a polypropylene glycol, an (ethylene oxide-propylene oxide) diol copolymer or a polytetrahydrofuran diol.
A polyester diol, such as a polyadipate diol, a poly(xcex5-caprolactone diol), a polyphthalate diol or a polycarbonate diol, may also be used as the functional polymer.
The functional polymer may also be an (ether-ester) diol copolymer, preferably comprising a central polyester moiety, for example a poly(xcex5-caprolactone), and two adjacent polyether moieties, for example a polytetrahydrofuran.
Trimerization catalysts according to the invention include tertiary amines which allow the catalysis of the reactions for forming isocyanurates and also the reactions between hydroxyl and isocyanate groups or water and isocyanate groups. Such catalysts include DMEA (dimethylethanolamine), TMBDA (tetramethylbutanediamine), alkylamino ethers (for example bis(dimethylaminoethyl) ether), piperazines such as piperidine, tertiary alkylamines such as 3dialkylaminopropionamides, TEA (triethylamine), N,N-dialkyl-3-(dialkylamino)propylamine, substituted morpholines such as N-acetamidopropylmorpholine, tris(dimethylamino)phenol and tris(dimethylaminomethyl)phenol, or metal salts such as dibutyltin dilaurate. Advantageously, tris(dimethylaminomethyl)phenol or dibutyltin dilaurate is used.
According to a further embodiment of the invention, the copolymer according to the invention containing isocyanurate groups combined with urea or urethane groups is the reaction product of the polyisocyanate with the functional polymer, with the trimerization catalyst and also with a chain extender comprising an aromatic diamine.
A mixture of monoamine and diamine which is of low viscosity at 20xc2x0 C., one or more of which are grafted onto a polyether block with a molecular mass of between 250 and 5,000 g/mol, and one or more diamines of low molecular weight may be used as a chain extender according to the invention.
The extender may be a mixture of 2,4 and 2,6 isomers of DETDA (diethyltoluenediamine), piperazine or DEDA (diethylenediamine), MEA (monoethanolamine), methylenebis(N,N-dibutyldianiline), IPDA (isophoronediamine), or a mixture of 3,5-dimethylthio-2,4-toluenediamine and -2,6-toluenediamine isomers.
Additional useful extenders include triols such as glycerol or trimethylol propane, polyesters or polyethers such as polyethylene adipate (PEA) or any other polyadipate, polypropylene glycol, polypropylene glycol diamine, polytetramethylene ether glycol (PTMEG) or polytetramethylene ether glycol diamine.
According to a further embodiment of the invention, the adhesive also comprises a hydrophobic or hydrophilic silica, in a mass fraction ranging from 0.01% to 1%.
In addition, the adhesive of the invention may comprise a diene elastomer, at least partly derived from butadiene, and/or a polyether comprising a silane group at each of its chain ends, in a mass fraction ranging from 1% to 50%. The diene elastomer, for example, a butadiene homopolymer or a butadiene-acrylonitrile copolymer, and the polyether containing a silane group are each used in liquid form (they are also known as xe2x80x9creactive liquid rubbersxe2x80x9d) to impart flexibility or a given suppleness to the adhesive according to the invention.
An example of a butadiene homopolymer which may be used is a polybutadiene containing hydroxyl end functional groups.
A butadiene-acrylonitrile copolymer may be used is an example of a copolymer containing amine end groups.
An example of a polyether containing silane groups which may be used is a polyoxypropylene containing silane end groups.
The process for preparing the adhesive comprises reacting polyisocyanate, functional polymer, trimerization catalyst and chain extender to obtain the copolymer usable as the adhesive in an amount of
a) between 60 and 70% of the polyisocyanate,
b) between 10 and 20% of the functional polymer;
c) 0.01 and 1% of the trimerization catalyst, and
d) between 0 and 20% of the chain extender, wherein the amount of reactant is expressed as % of mass fraction relative to the total mass of reagents.
It should be noted that this formulation in which the polyisocyanate is provided in excess makes it possible to compensate for the subsequent losses of this agent as a result of the humidity in ambient air and of the rubber surfaces with which the adhesive obtained contacts (for example casing surface or tread surface of a tire cover), and of migration of products originating from either of these rubber surfaces. Furthermore, the excess of polyisocyanate gives the copolymer obtained satisfactory resistance to the subsequent aminolysis reaction of the urea or urethane groups.
According to a further aspect of the invention, the preparation process involves combining the polyisocyanate with a blend comprising the functional polymer, the trimerization catalyst and the chain extender, wherein the blend has a viscosity, measured according to the Brookfield technique, of between 38 and 46 Poises.
It will be noted that the low viscosities for the polyisocyanate and for the blend give them a liquid state at ambient temperature, which makes the adhesive thus obtained easier to use at ambient temperature.
A tread of a tire cover according to the invention is prepared from a cross-linked rubber composition for the purpose of retreading, wherein the tread comprises the adhesive of the invention on its radially internal face.
A tire cover according to the invention is such that it comprises this tread.
An article according to the invention comprises two parts that are bonded together at two of their respective faces by means of the adhesive of the invention, wherein at least one of the faces comprises a cross-linked rubber composition. The adhesive is preferably applied to each of these two parts, for example using a brush or a spray gun.
The bonding by the adhesive is advantageously carried out at ambient temperature, i.e., between 20xc2x0 C. and 40xc2x0 C., by exerting on the two parts to be bonded a pressure which may range from 0.03 bar to 5 bar, the duration of the exertion being proportionately shorter the higher the pressure.
It will be noted that, in the case of a pressure of 0.03 bar, this pressure must be exerted immediately after applying the adhesive, whereas in the case of a pressure greater than or equal to 2 bar, it can be exerted at any time before the adhesive has gelled.
It will also be noted that a xe2x80x9cmaturationxe2x80x9d time is required for the assembly in the bonded state, in order for the activity of the adhesive to be ensured. This time is at least 48 hours at ambient temperature, or a few hours at a temperature of between 60xc2x0 C. and 100xc2x0 C.
According to an embodiment of the invention, each of the two faces is made from a cross-linked rubber composition, in particular a composition comprising predominantly at least one diene elastomer.
The term xe2x80x9cdiene elastomerxe2x80x9d means an elastomer derived at least in part (i.e., a homopolymer or a copolymer) from diene monomers (monomers bearing two conjugated or unconjugated carbonxe2x80x94carbon double bonds), in particular:
any homopolymer obtained by polymerization of a conjugated diene monomer containing from 4 to 12 carbon atoms;
any copolymer obtained by copolymerization of one or more dienes, conjugated together or with one or more vinyl aromatic compounds containing from 8 to 20 carbon atoms;
a ternary copolymer obtained by copolymerization of ethylene or of an alpha-olefin containing 3 to 6 carbon atoms with an unconjugated diene monomer containing from 6 to 12 carbon atoms, such as the elastomers obtained from ethylene or from propylene with an unconjugated diene monomer of the above-mentioned type, in particular, 1,4-hexadiene, ethylidenenorbomene or dicyclopentadiene; or
a copolymer of isobutene and of isoprene (butyl rubber or IIR), and also the halo, in particular chloro or bromo, versions of this type of copolymer.
Particularly preferred diene elastomers are chosen from the group consisting of polybutadienes (BR), polyisoprenes (IR) or natural rubber (NR), styrene-butadiene copolymers (SBR), terpolymers of ethylene, of propylene and of a diene (EPDM), butyl rubber and chloroprene.
In a further embodiment of the invention, one of the faces of the parts to be bonded together by the adhesive to obtain the above-mentioned article comprises a cross-linked rubber composition, while the other face comprises a ferrous metal or an iron-based metal alloy, such as steel.
According to a still further embodiment of the invention, one of the faces comprises a cross-linked rubber composition, while the other face comprises a synthetic fabric, such as a fabric of bi-elastic knit type to form membranes of configuration containing fibers sold under the registered trademark xe2x80x9cLYCRAxe2x80x9d.
According to a still further aspect of the invention, one of the faces comprises a cross-linked rubber composition, while the other face comprises a rigid plastic, such as a thermosetting polyurethane (for example, application of decoration to tire covers).
Before applying the adhesive according to the invention to one or each of the cross-linked rubber compositions, a chemical or physical modification of the surface of the corresponding composition is carried out, with the aim of making it compatible with the adhesive, so as to improve its adhesiveness and/or its wettability. This surface modification also makes it possible to remove the xe2x80x9ccontaminationxe2x80x9d layers or the layers of low cohesion.
Chemical modification of the surface may be carried out according to any one of the following techniques.
The surface of the cross-linked rubber composition may be treated with a solution of trichloroisocyanuric acid in a solvent (abbreviated as TIC, for example 3% in ethyl acetate). Treatment with this solution results in the creation of Cxe2x80x94Cl, Cxe2x80x94O and COOxe2x80x94 linkages at the surface of the cross-linked rubber compositions. It should be noted that TIC can promote the phenomenon of interdiffusion of the macromolecular chains of the adhesive with respect to those of the elastomers of the cross-linked rubber compositions.
Reference may be made to the following three articles for a detailed description of this chemical modification method:
Surface modification of synthetic vulcanized rubber, M. M. Pastor-Blas, M. S. Sanchez-Adsuar, J. M. Martin-Martinez, Polymer Surface Modification: Relevance to Adhesion, pp. 379-400, 1995.
Surface modification of synthetic vulcanized rubber, N. M. Pastor-Blas, M. S. Sanchez-Adsuar, J. M. Martin-Martinez, J. Adhesion Science Technologie, vol. 8, No. 10, pp. 1093-1114 (1994).
Halogenation of styrene-butadiene rubber to improve its adhesion to polyurethanes, Fernandez-Garcia, Orgiles-Barcelo and Martin-Martinez, J. of Sci. Technol., vol. 5, No. 12, pp. 1065-1080 (1991).
A solution of fumaric acid in an organic solvent, or an aqueous solution of sodium dichloroisocyanurate, or alternatively a solution sold by the company Kommerling under the name xe2x80x9cHalosol W5xe2x80x9d, may also be applied to the surface of the composition to be treated. Electrochemical methods may also be used, in order to give the assemblies made using the adhesive according to the invention a great increase in peel force.
The composition to be treated may also be immersed for a period ranging from 2 to 5 minutes in a bath of acidified bleach, based on 25 parts by weight of NaOCl (480 chlorometric) and 10 parts of HCl (d=1.19) per 1000 parts of demineralized water. The composition extracted from the said bath is then rinsed with mains water, after which it is oven-dried at 60xc2x0 C.
A physical modification of the surface state may be carried out, for example, according to techniques using plasmas (excited gases emitting light radiation, in particular in the ultraviolet range), most particularly the xe2x80x9cCorona dischargexe2x80x9d technique, by luminescent discharges, or alternatively using a plasma at atmospheric pressure.
These techniques make it possible to create polar groups (of carbonyl, carboxyl or hydroxyl type, for example) at the surface of the cross-linked rubber compositions, and do so independently of the nature of the gas used (plasmas based on oxygen or ammonia, in particular).
The above-mentioned characteristics of the present invention, along with others, will be understood more clearly on reading the following description of several Examples of the invention, which are given for illustrative purposes and are not intended to limit the invention.